Question

Lab: Exploration of Avogadro's Law-Determination of Percentage of Sodium Bicarbonate in an Alka-Seltzer Tablet

1) One concern with setting up a closed system experiment where gas is evolved during a reaction is the presence of leaks (another concern is that the system will explode if too much gas evolves!). We know that some leaking of pressure occurs because after a maximum pressure is achieved it slowly starts to decrease with time. Will a leak during gas evolution overstate or understate the amount of sodium bicarbonate in an Alka-Seltzer tablet and why?

2)In our rearranged Ideal Gas Law Δn=(ΔPV)/(RT) we considered V and T to remain constant. Is this a reasonable assumption and why? Assuming that T increased as a function of the reaction, would this cause an overestimation or underestimation of the amount of sodium bicarbonate?

3) To calculate the amount of sodium bicarbonate, we assume that there was enough citric acid to react completely with the sodium bicarbonate. However, if the citric acid is the limiting reagent, then the calculated amount for sodium bicarbonate would be incorrect. How could we experimentally test this possibility? Describe in two sentences how you would change the reaction conditions to account for this.

Answer 1

1) Answer , - Overestimation

Explanation :-

Our entire experiment is based on the measurement of pressure developedo due to the gas evolved ,during the reaction . The leakage in the closed system is markedly perceptible after certain maximum pressure built up during the experiment , even though it occurs throughout. This leakage of the gas, therefore , would result into recording a lower prressure in the system than the actual pressure built up.

Again , since pressure is inversely proportional to volume of the gas ( Boyle's law ) , the volume of the gas calculated would be more , hence it would cause an overestimation of the amount of sodium bicarbonate in the sample.   

2 ) Answer, - (i) Not a reasonable assumption.

..........................(ii) Under estimation.

Explanation :-

The ideal gas law relation PV = nRT, has been rewritten as

n = PV / RT ...................assuming that V and T both are constant

but when V & T both are fixed, how P would undergo a change ? , hence no need to use P . Therefore, it is not a reasonable assumption.

Further, the pressure measured would be greater than the actual pressure of the evolved gas due to the assumption that " T increases as a function of reaction " ,resulting into decrease in volume of the gas evolved. This situation would finally result into underestimation of the amount of sodium bicarbonate on subsequent calculations.

3) " If citric acid is the limiting reagent , then the calculated amount for sodium bicarbonate would be incorrect "

This can be tested experimentally by collecting the unreacted mass of the reagent and retreating with more citric acid , separately. The evolution of the gas again would prove the presence of unreacted sodium bicarbonate.

We should, therefore , account for this -

(i) by studying the stoichiometric relation of (Citric acid + Sodium bicarbonate ) reaction

(ii) Adjust the weight of sample taken accordingly..